The present invention relates generally to devices responsive to angular motion or rotation, and more particularly to a Sagnac interferometer for measuring the angular rate of rotation of a platform.
Rotation rate sensors are utilized in a variety of different applications including use as a rate gyroscope and a gyroscope test turn-table, as well as application to tachometers for generator speed control, inertial navigation and non-magnetic compasses. In its most common application, the device is disposed in a gimbal mounting and used in the manner of a gyroscope, stablized about one sensitive axis or about two or three mutually perpendicular sensitive axes.
It has been known for some time that the Sagnac interferometer can be used to detect the rotation rate of any rotating frame. The Sagnac interferometer is based on the existence of the measurable phase shifting effect of angular motion upon the transmission of counterpropagating electromagnetic waves in a light circuit loop path disposed in the plane of the angular motion.
Referring to FIG. 1, there is shown a typical prior art Sagnac interferometer. The assembly of FIG. 1 is mounted on a platform and is designed to sense the rotation rate of that platform. A beam of light, in this case a laser beam from a laser source 10, is split by a beamsplitter 12 into two beams diverging at right angles to each other. These two beams are then focused by means of the lenses 14 and 16 into the ends of a helically wound single mode optical fiber coil 18. The light focused by the lens 14 traverses the optical fiber 18 in a clockwise direction, while the light focused by the lens 16 traverses the light path circuit 18 in a counter-clockwise direction. When these two counterpropagating optical beams have traversed in their respective directions through the light path circuit 18, they will again impinge upon the beamsplitter 12 and will interfere with each other. This light interference will form a fringe or interference pattern. As the platform on which the optical fiber coil 18 is rotated, there will be a measurable intensity change in the light interference patterns obtained from the beamsplitter 12. This measurable change in intensity is due to the relative phase shift between the light propagating in the clockwise direction and the counter-clockwise direction in the optical fiber coil 18 due to rotation. This change in intensity is proportional to the phase shift between the two counterpropagating optical beams which, in turn, is proportional to the rotation rate in the plane of the optical fiber coil 18. Thus, it is possible to measure the rotation rate in the plane by measuring the optical intensity of either or both of the interference pattern outputs from the optical beamsplitter 12. To this end, a photodetector 20 is disposed to detect the interference pattern component 19 and thus to measure the intensity I.sub.3. Likewise, the interference pattern component 21 is directed by the beamsplitter 12 to a second beamsplitter 22 which directs a portion thereof to a photodetector 24 to measure intensity I.sub.4.
It is well known that the phase shift between the counterpropagating optical beams and thus the intensities I.sub.3 and I.sub.4 in the optical fiber coil 18 vary sinosoidally with the variation of the rotation rate of the plane in which the fiber optic coil rests. This variation of the intensities I.sub.3 measured at the photodetector 20 and I.sub.4 measured at the photodetector 24 caused by the phase shift variation is plotted in FIG. 2 versus the rotation rate of the optical fiber coil platform. It can be seen that the optical intensities I.sub.3 and I.sub.4 are 180 degrees out of phase.
A particular problem with the Sagnac interferometer is that the sensitivity of the device is directly proportional to the slope or derivative of the interferometer output intensities I.sub.3 and I.sub.4. Accordingly, as can be seen from FIG. 2, the sensitivity of the device will vary from zero to a maximum as the slope or the derivative of the interferometer output intensities varies from zero to a maximum. It can also be seen that the maximum sensitivity point 30 is at the quadrature or 90 degree phase shift differential point. Likewise, the sensitivity approaches zero as the rotation rate approaches zero. This can be understood mathematically by noting that the detected intensity is proportional to cos.sup.2 .phi., where 2.phi.=8.pi.NA.OMEGA./.lambda. c, .OMEGA. is the Sagnac phase shift, NA is the total area enclosed by the fiber, and .lambda. and c are the free-space wavelength and light velocity, respectively.
Thus, it can be seen that sensitivity of the device may vary with the rotation rate being sensed. Rotation rates near the zero rotation rate or rotation rates that provides a 180 degree phase shift between the counterpropagating beams will be sensed with minimum sensitivity.
As noted above, in order to obtain a high sensitivity at low rotation rates, the Sagnac interferometer must be operated at its maximum sensitivity or the quadrature point. Accordingly, it is necessary to introduce a non-reciprocal phase bias of .pi./2 between the counterpropagating beams via some external means. Such a .pi./2 phase shift will make the detected intensity proportional to sin 2.phi..apprxeq.2.phi. for small rotation rates. Such a non-reciprocal phase shift may be introduced by applying a magnetic field to a portion of the fiber in the interferometer to induce a phase change via the magneto-optic Faraday effect. However, such a Faraday effect phase shifter requires large electrical currents. Additionally, a Faraday effect phase shifter only introduces non-reciprocal phase shift to light which is circularly polarized. Thus, this device would require an additional set of optical elements (quarter-wave plates) to convert the linearly polarized signal to circular polarization, and then back again. Moreover, such Faraday effect phase shifters are bulky devices and are difficult to maintain at the 90 degree phase shift point because of temperature drift problems.
A non-reciprocal phase shift could also be introduced by an electrooptic phase shifter included as an integral part of the interferometer loop. However, such an electrooptic device must be operated on a pulsed basis. Since such pulsed phase shift operation will have a transit time in the microsecond range, it is difficult to determine the precise amplitude of the phase shift that will be obtained. It should also be noted, both with respect to the electrooptic phase shifter and the Faraday effect phase shifter, that the introduction of such additional elements into the fiber optic path can cause increased interferometer noise due to reflections and increased susceptibility to external perturbation effects.
Accordingly, it can be seen from the above that it would be highly desirable to introduce a phase shift in a reciprocal fashion to thereby eliminate the problems attendant to the use of Faraday cells and electrooptic devices.
However, even if a reciprocal phase shift is inserted into the system, the sensitivity of the system will still be dependent on the rotation rate being sensed. Thus, a system which is phase shifted in order to operate at the quadrature point will have very high sensitivity for very low rotation rates, but will have a decreasing sensitivity as the rotation rate and thus the phase shift induced in the optical beams increases.